Asymmetric membranes, which consist of a very thin, dense skin supported by a thicker, porous substructure layer, are used extensively in the reverse-osmosis desalination of brine. The technology for the formation of economically feasible asymmetric membranes for reverse osmosis was developed by Loeb and Sourirajan [Adv. Chem. Ser. 38, 117 (1962)] and continues to be improved.
Asymmetric membranes of polyquinoxalines have been employed in the separation of gaseous mixtures (U.S. Pat. No. 4,732,586).
While the literature is replete with descriptions of tablets, capsules and multiparticulates which deliver active substances by diffusion or osmotic pumping, none have taught the use of delivering active substances using a device with a coating comprised of an asymmetric membrane.